Thread: 3 phase motor question

It is 415V when connected in star, and 240V when in delta. Yes it will run off a single phase 240V VFD. But two observations:

- it is a 2 pole motor. If run from a VFD at 120Hz it would spin at 7200rpm. That could be a bit much for the rotor. Of course you could choose to let it only spin say 5000rpm at about 80Hz, but that way you give away a very useful part of the available speed range. A 4 pole motor would be a much better choice to be driven by a VFD.

- it is already a fairly old looking motor. A brand new motor of 3/4HP is only about $140 list price, often much cheaper on eBay. 3 phase motors are the simplest and cheapest to manufacture motors. I would wait and look out for a new or at least a "new old stock" offer.

That is an interesting observation.

I need to build a variable speed drive for a blower that is currently driven by a single phase 3/4 Hp.
The flow is regulated by a gate, but this way the blower goes full blast all the time and it is annoyingly noisy.
I understand that the single phase motors capacitor started do not work on a VFD
I wonder if I can use an old 3 phase 415 motor that I have (1/2 HP) and hook it up to a (single phase in 3 phase out) VFD.
Do I lose power?
What speed rate can I expect?
How do I know if it is a 2 pole or 4 pole?

> I need to build a variable speed drive for a blower that is currently driven by a single phase 3/4 Hp.
> The flow is regulated by a gate, but this way the blower goes full blast all the time and it is annoyingly noisy.
> I understand that the single phase motors capacitor started do not work on a VFD

Marc, a blower is much easyer to convert to variable speed than a machine tool. The reason is, that a fan requires only very little power at reduced speed. Assume a fan consumes 1HP at 3,000rpm, then at half that speed (or 1500rpm) it will only require 1/4HP, and at half that speed again (750rpm) only one quarter of 1/4HP (or 1/16HP).


Because of this, it is even possible to use a special, very low cost type of VFD. It is a VFD that it designed for just one purpose: to speed control single phase fan motors. It will only cover a speed range of 1:4, but that is usually perfectly adequate for HVAC applications. Key advantage is that you can keep your existing motor. And this being a metalworking forum: NO, it is useless for machine tools because it has absolutely no power at lower speeds. It is only usable for pumps and fans, and maybe for very low power things things like a scroll saw.


> I wonder if I can use an old 3 phase 415 motor that I have (1/2 HP) and hook it up to a (single phase in 3 phase out) VFD.

You sure can do that.

> Do I lose power?

If your blower is now fitted with a 3/4HP motor, I do not think you can get away with a 1/2HP, unless you would accept a slightly lower top speed. To be safe, better use a 3/4HP motor and VFD.

> What speed rate can I expect?

with a 1/2HP 3phase motor and VFD? I think you would have to accept a maximum airflow of 1/3 less than with a 3/4HP single phase constant speed motor.

> How do I know if it is a 2 pole or 4 pole?


If its 50Hz speed is around 3000rpm, it is a 2 pole motor. Blowers and fans usually use 2-pole motors.

If its 50Hz speed is around 1500rpm, it is a 4 pole motor. Higher number of poles are unlikely to be used for a blower application.

Hi,

Thanks for the detailed reply.

I just used that motor as an example, I am not thinking of purchasing it. It's just often I see motors with 415v listed but see that they can be configured for delta so was wondering if they would run on 240V. Now I know that they can but it alters that rpm significantly.

I need a motor for my drill press and the original motor is a two speed 415v 700/1410rpm motor. I have some photos of the data plate below.

I don't think this motor is going to run very well from a vfd? I would love to be proven wrong though,.

Now simonl is able to get hold of these motors on ebay. NEW 3 PHASE ELECTRIC MOTOR(2.2KW) | eBay

If I run one of these at 240v will the rpm still be suitable? I don't want high rpm. Around 1400rpm is what I am aiming for.

Here are the photos of the motor that is current on my drill press that if possible would like to run on the vfd.

P1010144.jpgP1010145.jpg

Thanks

Originally Posted by Marc

That is an interesting observation.

I need to build a variable speed drive for a blower that is currently driven by a single phase 3/4 Hp.

Hi Marc,

See if you can find a suitably rated variac. That's what I use on my furnace blower ( ex spa bubble blower, quiet as. )

Regards
Ray

Originally Posted by Com_VC

I need a motor for my drill press and the original motor is a two speed 415v 700/1410rpm motor. I have some photos of the data plate below.

I don't think this motor is going to run very well from a vfd? I would love to be proven wrong though,..............

Here are the photos of the motor that is current on my drill press that if possible would like to run on the vfd.

P1010144.jpgP1010145.jpg

This motor name plate shows it is a two speed motor which achieves the two speeds by switching the windings from delta configuration to double star. With this winding scheme each leg of the the delta configuration has two coils in series, and each leg of the star has two coils in parallel. Google "Dahlander winding" or "consequent pole motor" for more on this.

This type of motor will actually run on 240V (three phase) from a VFD, but with reduced speed and power. However it can not be "converted" to 240V operation simply by reconnecting the windings from star to delta in the usual way. I learned this the hard way with an Arboga mill, which has the same type of motor winding. Your other options are to get a 415V VFD (with 240V input is possible , but uncommon and costly); get a two speed 240V single phase motor; buy a standard three phase motor that can be hooked up in Delta to run on 240V through a VFD; or have your existing motor rewound to 240V (three phase) and run it on a VFD.

Regards,
Bill

This is a brochure of one of these VFD for single phase motors:
www.anaconsystems.com/data/opti_e2.pdf

and this is its user manual
www.inverterdrive.com/.../Invertek-Optidrive-E2

An alternative suitable for fans/pumps only, where the power requirement drops with the cube of the motor rpm. I believe these VFD's are marketed especially to HVAC customers and may be hard to buy from the usual channels.

Originally Posted by cba_melbourne

> I need to build a variable speed drive for a blower that is currently driven by a single phase 3/4 Hp.
> The flow is regulated by a gate, but this way the blower goes full blast all the time and it is annoyingly noisy.
> I understand that the single phase motors capacitor started do not work on a VFD

Marc, a blower is much easyer to convert to variable speed than a machine tool. The reason is, that a fan requires only very little power at reduced speed. Assume a fan consumes 1HP at 3,000rpm, then at half that speed (or 1500rpm) it will only require 1/4HP, and at half that speed again (750rpm) only one quarter of 1/4HP (or 1/16HP).


Because of this, it is even possible to use a special, very low cost type of VFD. It is a VFD that it designed for just one purpose: to speed control single phase fan motors. It will only cover a speed range of 1:4, but that is usually perfectly adequate for HVAC applications. Key advantage is that you can keep your existing motor. And this being a metalworking forum: NO, it is useless for machine tools because it has absolutely no power at lower speeds. It is only usable for pumps and fans, and maybe for very low power things things like a scroll saw.


> I wonder if I can use an old 3 phase 415 motor that I have (1/2 HP) and hook it up to a (single phase in 3 phase out) VFD.

You sure can do that.

> Do I lose power?

If your blower is now fitted with a 3/4HP motor, I do not think you can get away with a 1/2HP, unless you would accept a slightly lower top speed. To be safe, better use a 3/4HP motor and VFD.

> What speed rate can I expect?

with a 1/2HP 3phase motor and VFD? I think you would have to accept a maximum airflow of 1/3 less than with a 3/4HP single phase constant speed motor.

> How do I know if it is a 2 pole or 4 pole?


If its 50Hz speed is around 3000rpm, it is a 2 pole motor. Blowers and fans usually use 2-pole motors.

If its 50Hz speed is around 1500rpm, it is a 4 pole motor. Higher number of poles are unlikely to be used for a blower application.

Hi and thank you for the reply.

The forge blower and its motor come from a large side blast forge, far larger than required for the vertical blow forge I want to build. The motor I want to slow down or replace for one that can be made slower or variable is an ordinary capacitor starting (ancient, massive) 3/4 HP 1440 rpm 240 V.
The 3 phase motor I have is also 1400 revs and also ancient and massive so I gather it is a 4 pole motor.
The blower is pulley driven and I could simply reduce the size of the pulley to slow it down but it would be nice to be able to have a few different speed for the forge blower.

So is it possible to make the single phase motor variable speed? what happens to the centrifugal switch if it slows down too much?
If not the .5 3phase is still ample power and that one I can run on a VFS ?
Thanks again
Marc

Originally Posted by RayG

Hi Marc,

See if you can find a suitably rated variac. That's what I use on my furnace blower ( ex spa bubble blower, quiet as. )

Regards
Ray

I like "quite as", a lot
What is a variac?

Originally Posted by Marc

I like "quite as", a lot
What is a variac?

Hi Marc,

Variac is a brand name, but it's become the generic term for a variable auto-transformer, It might be a cheaper option than a VFD, this one is 1/2hp
0-260VAC Variable Laboratory Autotransformer (VARIAC) | eBay

Regards
Ray

> So is it possible to make the single phase motor variable speed? what happens to the centrifugal switch if it slows down too much?
No, as soon as there is a centrifugal switch it is not suitable for any kind of electronic speed control. A variac would probably work, but if you slow down enough to close the centrifugal switch, the start capacitor is likely to fry after a few minutes.

> If not the .5 3phase is still ample power and that one I can run on a VFS
Yes, that would be the best solution.

What rpm would be achieved when running the following motor wired for 240v delta. Is there a formula to work it out, I'm not even sure how many poles that motor has.

NEW 3 PHASE ELECTRIC MOTOR(2.2KW) | eBay

Thanks

Originally Posted by Com_VC

What rpm would be achieved when running the following motor wired for 240v delta. Is there a formula to work it out, I'm not even sure how many poles that motor has.

NEW 3 PHASE ELECTRIC MOTOR(2.2KW) | eBay

Thanks

Hi Com_VC,

Same rpm as when it's star connected, the motor rpm only depends on the number of poles and the AC frequency, minus a small percentage for slip (usually 2-4% depending on load)

The formula is RPM = 120 * Frequency / Number of Poles, so using that 4 pole motor as an example, a 50 Hz 4 pole motor is 50*120/4 = 1500 less 4% slip == 1440 RPM

Regards
Ray

Originally Posted by cba_melbourne

> So is it possible to make the single phase motor variable speed? what happens to the centrifugal switch if it slows down too much?
No, as soon as there is a centrifugal switch it is not suitable for any kind of electronic speed control. A variac would probably work, but if you slow down enough to close the centrifugal switch, the start capacitor is likely to fry after a few minutes.

> If not the .5 3phase is still ample power and that one I can run on a VFS
Yes, that would be the best solution.

Hi Chris,

Yes, that's a very good point, I was thinking it was a typical blower type motor (shaded pole or whatever), not cap start..

Regards
Ray